At present, in accordance with a Blu-ray disc (registered trademark) standard using a blue-violet semiconductor laser, it is possible to commercialize an optical disc having a recording density of about 100 GB even for consumer use. From now on, even in optical disks, it is desirable to increase the capacity exceeding 500 GB. However, in order to implement such a high density with an optical disk, there is a need of a high densification technology using a new method different from the high densification technology in the related art by shortening the wavelength and increasing the NA of the objective lens.
In research on next generation storage technology, a holographic recording technology for recording digital information using holography has attracted attention. As a hologram recording technology, for example, there is JP 2004-272268 A (Patent Document 1). The document discloses “multiplexing method and device of spatially multiplexing holograms by partially and spatially overlapping adjacent stacks of the holograms. Each stack may further have complete advantages of other multiplexing technologies, for example, angle multiplexing, wavelength multiplexing, phase sign multiplexing, peristropic multiplexing, or fractal-theory multiplexing. An amount equal to a beam waist of a signal beam recording a hologram divides individual stacks of the hologram. In a recovering period, a certain hologram and another hologram adjacent to the certain hologram are simultaneously read. A filter is arranged at the beam waist of the recovered data, so that the read adjacent hologram is not transferred to a camera plane. Otherwise, a recovering other than the desired recovering can be filtered by an angle filter on a middle plane in an optical system having a limited angle pass band.”
In addition, as a technique of performing multiplex-recording by controlling a phase of a signal beam in a recording period, for example, there is JP 2013-114716 A (Patent Document 2). The document discloses “at least two recording light sources 105 and 106 as semiconductor lasers and objective lenses 119 and 121 which condense a light flux from each of the recording light sources on an optical information recording medium 120 are provided, each recording light source outputs an intensity-modulated light flux, and when a recovering light flux is condensed on the optical information recording medium, recording is performed on the optical information recording medium in such a relationship that the light flux of each recording light source and the light flux of the recovering generate reflected beams having different phase differences which are different by almost 90 deg. Accordingly, an arbitrary complex amplitude of the light is recorded without using a phase modulator.”